One conventional fire and overheat sensor is known as a "thermal wire". This system senses a fire or overheat condition by thermal conduction from ambient to the center of a 1/16 inch diameter stainless steel tube. The sensing element may be a hydride which generates a gas as the temperature increases, the generated gas being sensed by a pressure switch. Alternatively the sensing element may be a salt or glass or a thermistor element which melts or changes resistance as temperature increases thus causing a change in an electrical resistivity vs. temperature characteristic of the sensing element.
Another conventional fire and overheat sensor employs a far-infrared optical detector to detect radiometric heat in combination with a two spectrum, far-near infrared fire detector.
However, for many high ambient temperature applications, such as jet aircraft engine nacelles, this latter type of system may not be useable in that the system typically has a maximum ambient temperature limitation of approximately 400.degree. F. This maximum ambient temperature limitation is due in large part to the maximum temperature limits of the sensor electronics.
The thermal wire type of system, which typically has a higher ambient temperature limitation, is, suitable for use in an engine nacelle. However, this type of system has a relatively slow response time. As reported by Delaney, "Fire Detection System Performance in USAF Aircraft", Tecnical Report AFAPL-TR-72-49, August 1972 this type of system furthermore may not detect as many as 40% of confirmed fires while exhibiting up to a 60% false alarm rate.
In U.S. Pat. Nos. 4,701,624, 4,691,196, 4,665,390 and 4,639,598, all of which are assigned to the assignee of this invention, there are described fire sensor systems which have overcome the problems inherent in the aforementioned thermal wire type of system. These systems accurately and rapidly detect the occurrence of a fire while also eliminating false alarms. A combination of these techniques has been disclosed in U.S. patent application Ser. No. 07/322,866 using an optical fiber transmission medium employing wavelengths less than 2.5 microns. However, in that these systems employ wavelengths of less than 2.5 microns it is difficult for them to be simultaneously employed for detecting overheat conditions in the 200.degree. C. in a radiometric fashion as described in U.S. Pat. No. 4,647,776, which is assigned to the assignee of this patent application.
It is thus an object of the invention to provide both a flame and heat sensing system that employs wavelengths of less than approximately 2.5 microns for flame detection while simultaneously detecting an overheat condition.
It is a further object of the invention to provide a flame and heat sensing system that employs wavelengths of less than 2.5 microns for flame detection while simultaneously detecting an overheat condition such that an actual flame condition is not required to generate an alarm condition.
It is a further object of the invention to provide a fiber optic flame detection system with a temperature measurement capability by employing a temperature dependent radiation transmission characteristic of a material that comprises an in-line optical element, the material being provided with optical radiation at a first and a second wavelength and the transmission response of the material at the two wavelengths being detected to determine the temperature.
It is also an object of the invention to provide signal processing circuitry such that a fire sensing function and an overheat sensing function do not interfere with one another even though these two functions may share the same fiber, detectors and circuitry.